The present invention relates to a control device for an active filter (active type filter) for absorbing a harmonic current generated from a load connected to an a.c. power source to remove the harmonic current flowing out from the a.c. power source.
FIG. 5 is a circuit diagram showing a prior art control device for an active filter disclosed, for example, on page 69 of a lecture and thesis series (Vol. 1) of a general conference of The Electric and Information Society in 1985. In the figure, reference numeral 1 denotes an a.c. power source, numeral 2 denotes a load connected to the a.c. power source 1 for generating a harmonic current, numeral 3 denotes a load current detector for detecting the load current I.sub.L of the load 2, and numeral 4 denotes an inverter connected to the a.c. power source 1 through a reactor 5 and also connected at its d.c. output side to a capacitor 6.
Numeral 7 denotes an a.c. current detector for detecting the a.c. current Ic of the inverter 4, and numeral 8 denotes a bandpass filter for removing a harmonic component from the output signal i.sub.L of the load current detector 3 to produce a basic wave component i.sub.L1.
Numeral 9 denotes a first adder which produces a harmonic component i.sub.cref of the difference between the output signal i.sub.L1 of the bandpass filter 8 and the output signal i.sub.L of the load current detector 3.
Numeral 10 denotes a second adder which produces the difference between the output signal i.sub.cref of the first adder 9 and the output signal i.sub.c of the a.c. current detector 7 of the inverter 4. Numeral 11 denotes a PWM circuit which pulse-width-modulation controls a switching element of the arm element of the inverter 4 in accordance with the output signal of the second adder 10.
The operation of the control device for the active filter constructed as described above will be described with reference to the signal waveform diagram shown in FIG. 6. A current of rectangular wave is assumed here as the current I.sub.L (i.sub.L) of the load 2, and this basic wave component i.sub.L1 is produced by the bandpass filter 9 which so operates as to produce only a basic wave component.
The harmonic component i.sub.cref of the load 2 is obtained by the calculation of i.sub.L1 '-i.sub.L as shown in FIG. 6(d). With the harmonic component i.sub.cref as a current reference the a.c. side current Ic of the inverter 4 is controlled to be momentarily traced by the PWM circuit 11 to feed the harmonic component generated from the load 2 to the inverter 4. As a result, only the basic wave current component similar to the basic wave component i.sub.L1 flows as as the a.c. current Is of the a.c. power source 1.
In case of 3-phase a.c. power sources, a 3-phase/3-phase converter is provided at the input unit of the bandpass filter 8 as shown by an equation (1), the component of the load current is separated to a valid power component i.sub.p and an invalid power component i.sub.q,the bandpass filter 8 is replaced with a low-pass filter, and it is general to employ a system for producing only the d.c. component. ##EQU1##
This is because when the basic wave component of the load current is converted by the conversion matrix of the equation (1) into a 2-phase, both the valid current component ip and the invalid current component iq become d.c. component. As a consequence, when the d.c. signal produced by the low-pass filter is 2-phase/3-phase converted by a reverse conversion matrix of the equation (1), 3-phase basic wave components are obtained.
However, when the conversion matrix is applied to a single-phase a.c. power source, if i.sub.LR =i.sub.LS and i.sub.LT =0 are, for example, satisfied, the basic component of a load current is formed by superposing an a.c. component having a frequency as large as twice of the frequency of the power source on a d.c. component, and it becomes very difficult to design the low-pass filter.
Since the prior art control device for the active filter is constituted as described above, when it is applied to a single-phase a.c. power source, if the degree of harmonic component is near that of a basic wave component, it is very difficult to design the bandpass filter 8. There are problems that the output signal i.sub.L1 of the bandpass filter 8 is displaced in its phase from the basic wave current of the actual load current, the amplitude of the signal is varied, or the frequency of the power source is altered, etc. Further, when a 3-phase/2-phase conversion matrix is similarly applied, it is very difficult as described above to design a low-pass filter after the 3-phase/2-phase conversion is conducted, and there is a problem that an error of the actual load current from the basic wave current is increased, etc.